Static Public Members

Detailed Description

The QImageReader class provides a format independent interface for reading images from files or other devices.

The most common way to read images is through QImage and QPixmap's constructors, or by calling QImage::load() and QPixmap::load(). QImageReader is a specialized class which gives you more control when reading images. For example, you can read an image into a specific size by calling setScaledSize(), and you can select a clip rect, effectively loading only parts of an image, by calling setClipRect(). Depending on the underlying support in the image format, this can save memory and speed up loading of images.

To read an image, you start by constructing a QImageReader object. Pass either a file name or a device pointer, and the image format to QImageReader's constructor. You can then set several options, such as the clip rect (by calling setClipRect()) and scaled size (by calling setScaledSize()). canRead() returns the image if the QImageReader can read the image (i.e., the image format is supported and the device is open for reading). Call read() to read the image.

If any error occurs when reading the image, read() will return a null QImage. You can then call error() to find the type of error that occurred, or errorString() to get a human readable description of what went wrong.

Note: QImageReader assumes exclusive control over the file or device that is assigned. Any attempts to modify the assigned file or device during the lifetime of the QImageReader object will yield undefined results.

QImageReader autodetects the image format by default, by looking at the provided (optional) format string, the file name suffix, and the data stream contents. You can enable or disable this feature, by calling setAutoDetectImageFormat().

High Resolution Versions of Images

It is possible to provide high resolution versions of images should a scaling between device pixels and device independent pixels be in effect.

The high resolution version is marked by the suffix @2x on the base name. The image read will have its device pixel ratio set to a value of 2.

This can be disabled by setting the environment variable QT_HIGHDPI_DISABLE_2X_IMAGE_LOADING.

Member Type Documentation

enum QImageReader::ImageReaderError

This enum describes the different types of errors that can occur when reading images with QImageReader.

Constant

Value

Description

QImageReader::FileNotFoundError

1

QImageReader was used with a file name, but not file was found with that name. This can also happen if the file name contained no extension, and the file with the correct extension is not supported by Qt.

QImageReader::DeviceError

2

QImageReader encountered a device error when reading the image. You can consult your particular device for more details on what went wrong.

QImageReader::UnsupportedFormatError

3

Qt does not support the requested image format.

QImageReader::InvalidDataError

4

The image data was invalid, and QImageReader was unable to read an image from it. The can happen if the image file is damaged.

QImageReader::UnknownError

0

An unknown error occurred. If you get this value after calling read(), it is most likely caused by a bug in QImageReader.

If the currently assigned device is a QFile, or if setFileName() has been called, this function returns the name of the file QImageReader reads from. Otherwise (i.e., if no device has been assigned or the device is not a QFile), an empty QString is returned.

float QImageReader::gamma() const

Returns the gamma level of the decoded image. If setGamma() has been called and gamma correction is supported it will return the gamma set. If gamma level is not supported by the image format, 0.0 is returned.

int QImageReader::loopCount() const

For image formats that support animation, this function returns the number of times the animation should loop. If this function returns -1, it can either mean the animation should loop forever, or that an error occurred. If an error occurred, canRead() will return false.

int QImageReader::nextImageDelay() const

For image formats that support animation, this function returns the number of milliseconds to wait until displaying the next frame in the animation. If the image format doesn't support animation, 0 is returned.

int QImageReader::quality() const

Reads an image from the device. On success, the image that was read is returned; otherwise, a null QImage is returned. You can then call error() to find the type of error that occurred, or errorString() to get a human readable description of the error.

For image formats that support animation, calling read() repeatedly will return the next frame. When all frames have been read, a null image will be returned.

Reads an image from the device into image, which must point to a QImage. Returns true on success; otherwise, returns false.

If image has same format and size as the image data that is about to be read, this function may not need to allocate a new image before reading. Because of this, it can be faster than the other read() overload, which always constructs a new image; especially when reading several images with the same format and size.

void QImageReader::setAutoDetectImageFormat(boolenabled)

If enabled is true, image format autodetection is enabled; otherwise, it is disabled. By default, autodetection is enabled.

QImageReader uses an extensive approach to detecting the image format; firstly, if you pass a file name to QImageReader, it will attempt to detect the file extension if the given file name does not point to an existing file, by appending supported default extensions to the given file name, one at a time. It then uses the following approach to detect the image format:

Image plugins are queried first, based on either the optional format string, or the file name suffix (if the source device is a file). No content detection is done at this stage. QImageReader will choose the first plugin that supports reading for this format.

If no plugin supports the image format, Qt's built-in handlers are checked based on either the optional format string, or the file name suffix.

If no capable plugins or built-in handlers are found, each plugin is tested by inspecting the content of the data stream.

If no plugins could detect the image format based on data contents, each built-in image handler is tested by inspecting the contents.

Finally, if all above approaches fail, QImageReader will report failure when trying to read the image.

By disabling image format autodetection, QImageReader will only query the plugins and built-in handlers based on the format string (i.e., no file name extensions are tested).

void QImageReader::setQuality(intquality)

Sets the quality setting of the image format to quality.

Some image formats, in particular lossy ones, entail a tradeoff between a) visual quality of the resulting image, and b) decoding execution time. This function sets the level of that tradeoff for image formats that support it.

In case of scaled image reading, the quality setting may also influence the tradeoff level between visual quality and execution speed of the scaling algorithm.

The value range of quality depends on the image format. For example, the "jpeg" format supports a quality range from 0 (low visual quality) to 100 (high visual quality).

Sets the scaled size of the image to size. The scaling is performed after the initial clip rect, but before the scaled clip rect is applied. The algorithm used for scaling depends on the image format. By default (i.e., if the image format does not support scaling), QImageReader will use QImage::scale() with Qt::SmoothScaling.

Returns the size of the image, without actually reading the image contents.

If the image format does not support this feature, this function returns an invalid size. Qt's built-in image handlers all support this feature, but custom image format plugins are not required to do so.

Different image formats support different options. Call this function to determine whether a certain option is supported by the current format. For example, the PNG format allows you to embed text into the image's metadata (see text()), and the BMP format allows you to determine the image's size without loading the whole image into memory (see size()).